GVariant Format Strings
GVariant Format Strings
Variable Argument Conversions
This page attempts to document how to perform variable argument conversions
with GVariant
.
Conversions occur according to format strings. A format string is a two-way
mapping between a single GVariant
value and one or more C values.
A conversion from C values into a GVariant
value is made using the
g_variant_new()
function. A conversion from a GVariant
into C
values is made using the g_variant_get()
function.
Syntax
This section exhaustively describes all possibilities for GVariant format strings. There are no valid forms of format strings other than those described here. Please note that the format string syntax is likely to expand in the future.
Valid format strings have one of the following forms:
- any type string
- a type string prefixed with a
@
&s
,&o
,&g
,^as
,^a&s
,^ao
,^a&o
,^ay
,^&ay
,^aay
or^a&ay
.- any format string, prefixed with an
m
- a sequence of zero or more format strings, concatenated and enclosed in parentheses
- an opening brace, followed by two format strings, followed by a closing brace (subject to the constraint that the first format string correspond to a type valid for use as the key type of a dictionary)
Symbols
The following table describes the rough meaning of symbols that may appear inside a GVariant format string. Each symbol is described in detail in its own section, including usage examples.
Symbol | Meaning |
---|---|
b , y , n , q , i , u , x , t , h , d |
Used for building or deconstructing boolean, byte and numeric types. See Numeric Types below. |
s , o , g |
Used for building or deconstructing string types. See Strings below. |
v |
Used for building or deconstructing variant types. See Variants below. |
a |
Used for building or deconstructing arrays. See Arrays below. |
m |
Used for building or deconstructing maybe types. See Maybe Types below. |
() |
Used for building or deconstructing tuples. See Tuples below. |
{} |
Used for building or deconstructing dictionary entries. See Dictionaries below. |
@ |
Used as a prefix for a GVariant type string (not a prefix for a format string, so as is a valid format string but @^as is not). Denotes that a pointer to a GVariant should be used in place of the normal C type or types. For g_variant_new() this means that you must pass a non-NULL (GVariant *) ; if it is a floating reference, ownership will be taken, as if by using g_variant_ref_sink() . For g_variant_get() this means that you must pass a pointer to a (GVariant *) for the value to be returned by reference or NULL to ignore the value. See GVariant * below. |
* , ? , r |
Exactly equivalent to @* , @? and r . Provided only for completeness so that all GVariant type strings can be used also as format strings. See GVariant * below. |
& |
Used as a prefix for a GVariant type string (not a prefix for a format string, so &s is a valid format string but & s“ is not). Denotes that a C pointer to serialised data should be used in place of the normal C type. See Pointers below. |
^ |
Used as a prefix on some specific types of format strings. See Convenience Conversions below. |
Numeric Types
Characters: b
, y
, n
, q
, i
, u
, x
, t
, h
, d
Variable argument conversions from numeric types work in the most obvious
way possible. Upon encountering one of these characters, g_variant_new()
takes the equivalent C type as an argument. g_variant_get()
takes a pointer
to the equivalent C type (or NULL
to ignore the value).
The equivalent C types are as follows:
Character | Equivalent C type |
---|---|
b |
gboolean |
y |
guchar |
n |
gint16 |
q |
guint16 |
i |
gint32 |
u |
guint32 |
x |
gint64 |
t |
guint64 |
h |
gint32 (handle) |
d |
gdouble |
Note that in C, small integer types in variable argument lists are promoted
up to int
or unsigned int
as appropriate, and read back accordingly. int
is 32 bits on every platform on which GLib is currently supported. This
means that you can use C expressions of type int
with g_variant_new()
and
format characters b
, y
, n
, q
, i
, u
and h
. Specifically, you
can use integer literals with these characters.
When using the x
and t
characters, you must ensure that the value that
you provide is 64 bit. This means that you should use a cast or make use of
the G_GINT64_CONSTANT
or G_GUINT64_CONSTANT
macros.
No type promotion occurs when using g_variant_get()
since it operates with
pointers. The pointers must always point to a memory region of exactly the
correct size.
Examples
GVariant *value1, *value2, *value3, *value4;
value1 = g_variant_new ("y", 200);
value2 = g_variant_new ("b", TRUE);
value3 = g_variant_new ("d", 37.5):
value4 = g_variant_new ("x", G_GINT64_CONSTANT (998877665544332211));
{
gdouble floating;
gboolean truth;
gint64 bignum;
g_variant_get (value1, "y", NULL); /* ignore the value. */
g_variant_get (value2, "b", &truth);
g_variant_get (value3, "d", &floating);
g_variant_get (value4, "x", &bignum);
}
Strings
Characters: s
, o
, g
String conversions occur to and from standard nul-terminated C strings. Upon
encountering an s
, o
or g
in a format string, g_variant_new()
takes a
(const gchar *)
and makes a copy of it. NULL
is not a valid string; use
maybe types to encode that. If the o
or g
characters are used, care must
be taken to ensure that the passed string is a valid D-Bus object path or
D-Bus type signature, respectively.
Upon encountering s
, o
or g
, g_variant_get()
takes a pointer to a
(gchar *)
(ie: (gchar **)
) and sets it to a newly-allocated copy of the
string. It is appropriate to free this copy using g_free()
. NULL
may
also be passed to indicate that the value of the string should be ignored
(in which case no copy is made).
Examples
GVariant *value1, *value2, *value3;
value1 = g_variant_new ("s", "hello world!");
value2 = g_variant_new ("o", "/must/be/a/valid/path");
value3 = g_variant_new ("g", "iias");
#if 0
g_variant_new ("s", NULL); /* not valid: NULL is not a string. */
#endif
{
gchar *result;
g_variant_get (value1, "s", &result);
g_print ("It was '%s'\n", result);
g_free (result);
}
Variants
Characters: v
Upon encountering a v
, g_variant_new()
takes a (GVariant *)
. The value
of the GVariant
is used as the contents of the variant value.
Upon encountering a v
, g_variant_get()
takes a pointer to a (GVariant
*)
(ie: (GVariant **)
). It is set to a new reference to a GVariant
instance containing the contents of the variant value. It is appropriate to
free this reference using g_variant_unref()
. NULL
may also be passed to
indicate that the value should be ignored (in which case no new reference is created).
Examples
GVariant *x, *y;
/* the following two lines are equivalent: */
x = g_variant_new ("v", y);
x = g_variant_new_variant (y);
/* as are these: */
g_variant_get (x, "v", &y);
y = g_variant_get_variant (x);
Arrays
Characters: a
Upon encountering an a
character followed by a type string,
g_variant_new()
will take a (GVariantBuilder *)
that has been created as
an array builder for an array of the type given in the type string. The
builder will have g_variant_builder_end()
called on it and the result will
be used as the value. As a special exception, if the given type string is a
definite type, then NULL
may be given to mean an empty array of that type.
Upon encountering an a
character followed by a type string,
g_variant_get()
will take a pointer to a (GVariantIter *)
(ie:
(GVariantIter **)
). A new heap-allocated iterator is created and returned,
initialised for iterating over the elements of the array. This iterator
should be freed when you are done with it, using g_variant_iter_free()
.
NULL
may also be given to indicate that the value of the array should be ignored.
Examples
GVariantBuilder *builder;
GVariant *value;
builder = g_variant_builder_new (G_VARIANT_TYPE ("as"));
g_variant_builder_add (builder, "s", "when");
g_variant_builder_add (builder, "s", "in");
g_variant_builder_add (builder, "s", "the");
g_variant_builder_add (builder, "s", "course");
value = g_variant_new ("as", builder);
g_variant_builder_unref (builder);
{
GVariantIter *iter;
gchar *str;
g_variant_get (value, "as", &iter);
while (g_variant_iter_loop (iter, "s", &str))
g_print ("%s\n", str);
g_variant_iter_free (iter);
}
g_variant_unref (value);
Maybe Types
Characters: m
Maybe types are handled in two separate ways depending on the format string
that follows the m
. The method that is used currently depends entirely on
the character immediately following the m
.
The first way is used with format strings starting with a
, s
, o
, g
,
v
, @
, *
, ?
, r
, &
, or ^
. In all of these cases, for non-maybe
types, g_variant_new()
takes a pointer to a non-NULL
value and
g_variant_get()
returns (by reference) a non-NULL
pointer. When any of
these format strings are prefixed with an m
, the type of arguments that
are collected does not change in any way, but NULL
becomes a permissible
value, to indicate the Nothing case.
Note that the “special exception” introduced in the array section for
constructing empty arrays is ignored here. Using a NULL
pointer with the
format string mas
constructs the Nothing value — not an empty array.
The second way is used with all other format strings. For g_variant_new()
an additional gboolean argument is collected and for g_variant_get()
an
additional (gboolean *)
. Following this argument, the arguments that are
normally collected for the equivalent non-maybe type will be collected.
If FALSE
is given to g_variant_new()
then the Nothing value is
constructed and the collected arguments are ignored. Otherwise (if TRUE
was given), the arguments are used in the normal way to create the Just value.
If NULL
is given to g_variant_get()
then the value is ignored. If a
non-NULL
pointer is given then it is used to return by reference whether
the value was Just. In the case that the value was Just, the gboolean
will
be set to TRUE
and the value will be stored in the arguments in the usual
way. In the case that the value was Nothing, the gboolean
will be set to
FALSE
and the arguments will be collected in the normal way but have their
values set to binary zero.
Examples
GVariant *value1, *value2, *value3, *value4, *value5, *value6;
value1 = g_variant_new ("ms", "Hello world");
value2 = g_variant_new ("ms", NULL);
value3 = g_variant_new ("(m(ii)s)", TRUE, 123, 456, "Done");
value4 = g_variant_new ("(m(ii)s)", FALSE, -1, -1, "Done"); /* both '-1' are ignored. */
value5 = g_variant_new ("(m@(ii)s)", NULL, "Done");
{
GVariant *contents;
const gchar *cstr;
gboolean just;
gint32 x, y;
gchar *str;
g_variant_get (value1, "ms", &str);
if (str != NULL)
g_print ("str: %s\n", str);
else
g_print ("it was null\n");
g_free (str);
g_variant_get (value2, "m&s", &cstr);
if (cstr != NULL)
g_print ("str: %s\n", cstr);
else
g_print ("it was null\n");
/* don't free 'cstr' */
/* NULL passed for the gboolean *, but two 'gint32 *' still collected */
g_variant_get (value3, "(m(ii)s)", NULL, NULL, NULL, &str);
g_print ("string is %s\n", str);
g_free (str);
/* note: &s used, so g_free() not needed */
g_variant_get (value4, "(m(ii)&s)", &just, &x, &y, &cstr);
if (just)
g_print ("it was (%d, %d)\n", x, y);
else
g_print ("it was null\n");
g_print ("string is %s\n", cstr);
/* don't free 'cstr' */
g_variant_get (value5, "(m*s)", &contents, NULL); /* ignore the string. */
if (contents != NULL)
{
g_variant_get (contents, "(ii)", &x, &y);
g_print ("it was (%d, %d)\n", x, y);
g_variant_unref (contents);
}
else
g_print ("it was null\n");
}
Tuples
Characters: ()
Tuples are handled by handling each item in the tuple, in sequence. Each item is handled in the usual way.
Examples
GVariant *value1, *value2;
value1 = g_variant_new ("(s(ii))", "Hello", 55, 77);
value2 = g_variant_new ("()");
{
gchar *string;
gint x, y;
g_variant_get (value1, "(s(ii))", &string, &x, &y);
g_print ("%s, %d, %d\n", string, x, y);
g_free (string);
g_variant_get (value2, "()"); /* do nothing... */
}
GVariant *
Characters: @
, *
, ?
, r
Upon encountering a @
in front of a type string, g_variant_new()
takes a
non-NULL
pointer to a GVariant
and uses its value directly instead of
collecting arguments to create the value. The provided GVariant
must have
a type that matches the type string following the @
. *
is the same as
@*
(ie: take a GVariant
of any type). ?
is the same as @?
(ie: take
a GVariant
of any basic type). r
is the same as r
(ie: take a
GVariant
of any tuple type).
Upon encountering a @
in front of a type string, g_variant_get()
takes a
pointer to a (GVariant *)
(ie: a (GVariant **)
) and sets it to a new
reference to a GVariant
containing the value (instead of deconstructing
the value into C types in the usual way). NULL
can be given to ignore the
value. *
, ?
and r
are handled in a way analogous to what is stated above.
You can always use *
as an alternative to ?
, r
or any use of @
.
Using the other characters where possible is recommended, however, due to
the improvements in type safety and code self-documentation.
Examples
GVariant *value1, *value2;
value1 = g_variant_new ("(i@ii)", 44, g_variant_new_int32 (55), 66);
/* note: consumes floating reference count on 'value1' */
value2 = g_variant_new ("(@(iii)*)", value1, g_variant_new_string ("foo"));
{
const gchar *string;
GVariant *tmp;
gsize length;
gint x, y, z;
g_variant_get (value2, "((iii)*)", &x, &y, &z, &tmp);
string = g_variant_get_string (tmp, &length);
g_print ("it is %d %d %d %s (length=%d)\n", x, y, z, string, (int) length);
g_variant_unref (tmp);
/* quick way to skip all the values in a tuple */
g_variant_get (value2, "(rs)", NULL, &string); /* or "(@(iii)s)" */
g_print ("i only got the string: %s\n", string);
g_free (string);
}
Dictionaries
Characters: {}
Dictionary entries are handled by handling first the key, then the value. Each is handled in the usual way.
Examples
GVariantBuilder *b;
GVariant *dict;
b = g_variant_builder_new (G_VARIANT_TYPE ("a{sv}"));
g_variant_builder_add (b, "{sv}", "name", g_variant_new_string ("foo"));
g_variant_builder_add (b, "{sv}", "timeout", g_variant_new_int32 (10));
dict = g_variant_builder_end (b);
To extract data from nested dictionaries you can go through a vardict.
Examples
GVariant *data;
gint value = 1;
gint max = 3;
/* type (oa{sa{sv}) */
data = g_variant_new_parsed ("(%o, {'brightness': {'value': <%i>, 'max': <%i>}})",
"/object/path", value, max);
{
GVariant *params;
GVariant *p_brightness;
gchar *obj
gint p_max;
g_variant_get (data, "(o@a{?*})", &obj, ¶ms);
g_print ("object_path: %s\n", obj);
p_brightness = g_variant_lookup_value (params, "brightness", G_VARIANT_TYPE_VARDICT);
g_variant_lookup (p_brightness, "max", "i", &p_max);
g_print ("max: %d\n", p_max);
}
Pointers
Characters: &
The &
character is used to indicate that serialised data should be
directly exchanged via a pointer.
Currently, the only use for this character is when it is applied to a string
(ie: &s
, &o
or &g
). For g_variant_new()
this has absolutely no
effect. The string is collected and duplicated normally. For
g_variant_get()
it means that instead of creating a newly allocated copy
of the string, a pointer to the serialised data is returned. This pointer
should not be freed. Validity checks are performed to ensure that the string
data will always be properly nul-terminated.
Examples
{
const gchar *str;
GVariant *value;
value = g_variant_new ("&s", "hello world");
g_variant_get (value, "&s", &str);
g_print ("string is: %s\n", str);
/* no need to free str */
}
Convenience Conversions
Characters: ^
The ^
character currently supports conversion to and from bytestrings or
to and from arrays of strings or bytestrings. It does not support byte
arrays. It has a number of forms.
In all forms, when used with g_variant_new()
one pointer value is
collected from the variable arguments and passed to a function (as given in
the table below). The result of that function is used as the value for this
position. When used with g_variant_get()
one pointer value is produced by
using the function (given in the table) and returned by reference.
Conversion | Used with g_variant_new() |
Used with g_variant_get() |
---|---|---|
^as |
equivalent to g_variant_new_strv() |
equivalent to g_variant_dup_strv() |
^a&s |
equivalent to g_variant_get_strv() |
|
^ao |
equivalent to g_variant_new_objv() |
equivalent to g_variant_dup_objv() |
^a&o |
equivalent to g_variant_get_objv() |
|
^ay |
equivalent to g_variant_new_bytestring() |
equivalent to g_variant_dup_bytestring() |
^&ay |
equivalent to g_variant_get_bytestring() |
|
^aay |
equivalent to g_variant_new_bytestring_array() |
equivalent to g_variant_dup_bytestring_array() |
^a&ay |
equivalent to g_variant_get_bytestring_array() |